The present invention relates to a temperature-dependent switch which comprises on the outside of its housing a first and at least a second connecting surface for directly connecting feed lines and, in the housing, a temperature-dependent switching mechanism, which, depending on its temperature, closes or opens an electrically conducting connection between the two connecting surfaces, wherein feed lines at their inner ends are directly connected to the connecting surfaces, the switch being encased by an insulating protective layer and the feed lines, at their free ends which are remote from the inner ends, being free from the protective layer.
Such a temperature-dependent switch is known from DE 41 39 091 C2.
Such temperature-dependent switches are frequently known from the prior art. They are used for protecting electrical appliances, such as hairdryers, motors for lye pumps, irons etc. from overheating and/or from an excessively high current.
For this purpose, the known temperature-dependent switches are connected to appliance to be protected such that they are arranged electrically in series with the appliance in the supply circuit thereof, with the result that the operating current of the appliance to be protected flows through the temperature-dependent switch. In addition, the switch is fitted to the appliance to be protected in such a way that it is brought to the same temperature as the appliance to be protected.
The known temperature-dependent switches comprise a temperature-dependent switching mechanism, which opens or closes an electrical connection depending on its temperature between two connecting surfaces provided on the outside on the housing of the switch. For this purpose, as a rule, a bimetallic part is provided in the switching mechanism, said bimetallic part being deformed suddenly from its low-temperature position into its high-temperature position when its switching temperature is reached, thereby, as a rule, lifting a movable contact part off from a fixed contact part.
The fixed contact part is connected to one of the two connecting surfaces, while the movable contact part interacts with the second connecting surface, either via the bimetallic part or a snap-action-disc or -spring associated with the bimetallic part.
Designs are also known in which the bimetallic part carries a contact bridge, which produces, directly, an electrical connection between two connecting surfaces.
Examples of such temperature-dependent switches are disclosed in DE 21 21 802 A, DE 26 44 411 A, DE 196 23 570, DE 103 01 803, DE 92 14 543 U, DE 91 02 841 U, DE 197 05 441 A1, DE 195 45 996 A1 or DE 10 205 001 371 A1 and other industrial property rights held by the applicant, such that reference may be made to these industrial property rights for further details.
When using the known switches, it is necessary to ensure, inter alia, that the switches are electrically insulated from the electrical appliance to be protected, such that undesirable short circuits do not occur.
Namely, the known switches often have an electrically conducting housing lower part, which is in the form of a pot and houses the temperature-dependent switching mechanism. The electrically conducting housing lower part is closed off by a likewise electrically conducting cover part, which is fixed on the housing lower part with an insulating film interposed. The first connecting surface is provided on the cover part, while the second connecting surface is provided on the base, the side wall or that edge of the housing lower part which holds the cover part.
Feed lines, generally either flexible connecting strand wires or rigid connecting lugs, are now galvanically or directly connected, generally connected by material-connecting engagement, i.e. usually soldered or welded, to these two connecting surfaces, the strand wires or connecting lugs then being used for the further wiring of the known temperature-dependent switches.
The switches which are prefabricated and provided with strand wires or connecting lugs in this way are then provided with a cap in order to insulate the switches electrically from the outside. If the switches have been provided with connecting lugs, the caps have corresponding slots, through which the connecting lugs need to be threaded when the cap is plugged onto the switch, which is not only correspondingly time-consuming and laborious, but always also involves the risk of the galvanic connection between the connecting lugs and the connecting surfaces being damaged or of the connecting lugs being bent, with the result that said connecting lugs are not suitable for subsequent automatic installation in the electrical appliances to be protected, but need to be further-processed.
If, on the other hand, the feed lines are in the form of strand wires, the switches are provided with so-called shrink-fit caps, which are sealed at one end, with the result that, once the shrink-fit caps have been plugged onto the switches which have been prefabricated with the strand wires, the strand wires protrude out of the shrink-fit cap at the other end. The shrink-fit caps are then shrunk onto the switch.
In the case of the switch known from DE 41 39 091 C2, which was mentioned at the outset, the feed lines are in the form of relatively rigid metal sheets, which are riveted, with their inner limbs, to the connecting surfaces. Then, in one embodiment, the switch with the riveted joints and the inner ends is encapsulated by injection moulding with a low-pressure epoxy resin in a low-pressure process at a tool temperature of from 150 to 180° C. The free ends of the metal sheets which are remote from the inner ends in this case remain free of epoxy resin. Once the epoxy resin has cured, connecting strand wires are soldered to the free ends of the metal sheets and the free ends are then bent over the inner ends.
By virtue of the riveting and the encapsulation by injection moulding with the thermosetting plastic, the intention is to ensure a fixed connection which is capable of permanently withstanding the mechanical loads between the metal sheets and the housing of the switch on which the connecting surfaces are formed. The encapsulation by injection moulding in this case also ensures good electrical insulation and sealing of the riveted joints, with the result that it is not possible for any dirt such as dust or liquids to enter the housing.
With the known switch, however, one disadvantage is that the riveting of the metal sheets is time-consuming and involves the risk of the housing being deformed during the riveting process. As a result of the extremely small dimensions of the temperature-dependent switches, however, it is possible for very small deformations of the housing to result in the switch no longer closing and/or opening reliably.
In addition, the known switch has a complex design and is complex to assemble owing to the additional metal sheets provided between the housing and strand wires. In order to connect each connecting strand wire, a riveting operation and, subsequently, a soldering operation and, thereupon, a bending operation are required.
Finally, the known switch can be used only to a restricted extent, since it does not provide any possibilities for a plug-type connection. The connecting strand wires used in the known switch still need to be soldered to the appliance to be protected, which is time-consuming and involves the risk of an insufficient “cold” soldered joint.
A connection technique with plug-type connections is demanded, however, by a large number of processors of the known temperature-dependent switches precisely because switches with such connections are fitted to the appliance to be protected simply, quickly and primarily reliably, to which a contribution is also made by the matching dimensions and interspaces in the plug-type connections, on the one hand, and the respective applications, on the other hand.
As has already been mentioned at the outset, it is already known to provide temperature-dependent switches directly with plug-type connections, which can be connected to the appliance to be protected by being screwed, by suitable clamping techniques or by being plugged on, for example. Owing to the complicated connection between the plug-type connections and the housing of the respective temperature-dependent switch and the required insulating caps or encapsulating housings, these switches are also complex to assemble and have the abovementioned disadvantages.
One particular disadvantage here is that the caps or encapsulating housings either have a very complicated design or else the fitting of the cap to the switch which has already been provided with connecting lugs is complex and therefore cannot be automated.
Such a temperature-dependent switch with soldered or welded plug-type connections is known from DE 92 14 544 U1.
DE 80 28 913 U1 discloses a temperature-dependent switch inserted into a two-part insolating housing made from thermoplastic material. The two housing parts are connected to one another by ultra sonic welding. This document explicitly mentions that a protective layer made from sintered epoxy resin is neither mechanically nor thermally stable and tends to crack especially under high pressure.